The disclosure of Japanese Patent Application No. 2000-191131 filed on Jun. 26, 2000, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of Invention
The invention relates to a behavior control apparatus and method for a motor vehicle, such as an automobile, or the like. More particularly, the invention relates to such a behavior control apparatus and method for controlling vehicle behavior through control of braking force.
2. Description of Related Art
As one type of behavior control apparatus of a vehicle, such as an automobile, or the like, a braking force control-type behavior control apparatus as disclosed in U.S. Pat. No. 5,813,732 has been well known in the art. When a vehicle undergoes an unfavorable or undesirable behavior, such as a spin or a drift-out, the behavior control apparatus of this type operates to apply braking force to a certain wheel or wheels, so as to suppress or prevent the undesirable behavior.
More specifically, when the vehicle behaves in an undesirable manner, for example, when the vehicle is brought into a spin state or a drift-out state, the behavior control apparatus of the above type operates to automatically apply braking force to a suitable wheel or wheels so as to decelerate the vehicle or apply a yaw moment to the vehicle in such a direction as to suppress the undesirable behavior. Thus, the control apparatus automatically suppresses the undesirable behavior without requiring any special maneuver to be performed by a vehicle operator or driver.
In the conventional braking force control-type behavior control apparatus as described above, the braking force applied to a suitable wheel or wheels is generally controlled in accordance with the degree of the undesirable behavior. Therefore, in a situation where the vehicle behavior suddenly deteriorates, a large amount of braking force is rapidly applied to the wheel or wheels in question, and then the braking force is relatively rapidly reduced as the vehicle behavior stabilizes. Hence, the deceleration of the vehicle sharply increases, and then relatively sharply decreases, resulting in pitching of the vehicle, which may cause a passenger of the vehicle to feel uncomfortable.
To solve the above-described problem, the braking force applied to a suitable wheel or wheels in accordance with the degree of the undesirable behavior may be controlled to a reduced value by, for example, setting a control gain to a relatively low value. In this case, however, the effect of the behavior control is reduced, and the undesirable behavior of the vehicle cannot be effectively and surely suppressed or prevented.
It is therefore an object of the invention to provide a braking force control type vehicle behavior control apparatus which is able to prevent a sharp reduction in the braking force that was increased to a large magnitude under behavior control, and thereby effectively and reliably prevent pitching of the vehicle that would be otherwise caused by the behavior control, without sacrificing the effect of the behavior control.
To accomplish the above and/or other object(s), one aspect of the invention provides a behavior control apparatus for controlling a behavior of a vehicle, in which a controller applies a braking force to at least one predetermined wheel of the vehicle when the vehicle undergoes an undesirable behavior, and controls a rate of reduction of the braking force to be smaller with an increase in a degree of deceleration of the vehicle caused by application of the braking force to the at least one predetermined wheel.
With the control apparatus constructed as described above, the rate of reduction of the braking force is reduced when the degree of deceleration of the vehicle is relatively high, as compared with when the degree of deceleration is relatively low. It is therefore possible to effectively and surely prevent pitching of the vehicle, which would otherwise occur due to a sharp reduction in the braking force that previously was increased by a large degree under behavior control. Also, the braking force applied to the predetermined wheel under behavior control is not reduced, and therefore the undesirable behavior of the vehicle can be effectively and surely suppressed or prevented.
In one preferred embodiment of the invention, the controller sets an upper limit value of the rate of reduction of the braking force to a smaller value as the braking force applied to the predetermined wheel increases, thereby to reduce the rate of reduction of the braking force.
In the above embodiment, the upper limit value of the rate of reduction of the braking force is set to a smaller value as the braking force applied to the predetermined wheel increases, so that the rate of reduction of the braking force is reduced. It is thus possible to reduce the rate of reduction of the braking force in accordance with the braking force applied to the predetermined wheel, without requiring calculation of the deceleration of the vehicle. Also, since the upper limit value of the rate of reduction of the braking force increases with a reduction in the braking force, the rate of reduction of the braking force gradually increases as the braking force decreases. Accordingly, in the course of reduction of the braking force applied to the predetermined wheel, the application of the braking force under the behavior control can be finished sooner, as compared with the case where the braking force applied to the wheel is reduced at a fixed or constant small rate.